1. Field of the Invention
Aspects of the present invention relate to a proton conducting titanate, a polymer nano-composite membrane including the same, and a fuel cell adopting the polymer nano-composite membrane, and more particularly, to a proton conducting titanate, a polymer nano-composite membrane including the proton conducting titanate and having reduced permeability to water and methanol and improved thermal stability, and a fuel cell having improved energy density and fuel efficiency by adopting the polymer nano-composite membrane.
2. Description of the Related Art
Direct methanol fuel cells (DMFC) using methanol solution as liquid fuel are a source of future clean energy that can replace fossil energy. Also, a DMFC is workable at room temperature and can be miniaturized and sealed. Thus, the DMFC has a very wide applicability in fields including zero-emission vehicles, home power generating systems, mobile communication equipment, medical appliances, military equipment, and aerospace industrial equipment, as well as in the field of portable electrical devices.
The DMFC is a power generating system that produces direct current by an electrochemical reaction of methanol and oxygen. A basic structure of the DMFC is shown in FIG. 1. Referring to FIG. 1, the DMFC includes a proton conducting membrane 11 interposed between an anode and a cathode.
The proton conducting membrane 11 is mainly made of a solid polymer electrolyte. The anode and the cathode include catalyst layers 12 and 13 formed on a cathode supporting layer 14 and an anode supporting layer 15, respectively. The supporting layers 14 and 15 are made of carbon cloth or carbon paper and their surfaces are treated so that water to be transferred to the proton conducting membrane 11 and water generated by the reaction at the cathode can easily pass therethrough while supplying a reaction gas or liquid. In FIG. 1, a reference numeral 16 represents a bipolar plate having grooves for injecting gas and which acts as a current collector.
When a reaction fuel is supplied to the DMFC having the above-described structure, an oxidation reaction occurs in the anode, wherein methanol and water are converted into carbon dioxide, protons, and electrons. During this process, the protons are transferred to the cathode via the proton conducting membrane 11.
Meanwhile, a reduction reaction occurs in the cathode, wherein oxygen molecules from the air receive the electrons from the anode and are converted into oxygen ions. Then, the oxygen ions react with the protons from the anode to produce water molecules.
In the above DMFC, the proton conducting membrane 11 is a solid polymer membrane and acts as separator of fuels to be supplied to each of the anode and the cathode while transferring the protons produced in the anode to the cathode.
A commercially available perfluorosulfonic acid membrane, sold under the name NAFION (a registered trademark of Dupont, Wilmington, Del.), is commonly used as the solid polymer membrane. It is known that since the solid polymer membrane is formed of a polymer that has a hydrophobic backbone and hydrophilic group-containing side chains, it can contain water. The protons migrate through a cluster formed by water that is contained in the solid polymer membrane. Therefore, it is preferable that in order to effectively transfer the protons, solid polymer membranes having an increased content of water to increase proton conductivity be used.
In a DMFC using an aqueous methanol solution as fuel, swelling of a solid polymer membrane may occur depending on the concentration of methanol in the aqueous methanol solution. Due to the swelling phenomenon, fuel that is not oxidized by an electrochemical reaction migrates from the anode to the cathode through the solid polymer membrane, thereby lowering the performance by creating a mixed potential in the cathode and also by wasting the fuel.
In order to address the above problem, it is desirable to develop an improved solid polymer membrane for DMFCs.
In forming a solid polymer membrane for DMFCs, a method of lowering the permeability of aqueous methanol solution by using rigid and heat-resistant polymers was proposed in U.S. Pat. No. 6,510,047. This method can remarkably lower the permeability of methanol. However, since this method also considerably reduces the ion conductivity of the polymer membrane, the performance, such as power density, of the fuel cell is considerably lowered.
As another method for forming a solid polymer membrane for DMFCs, a method of dispersing an inorganic filler in a polymer membrane was proposed in U.S. Pat. No. 6,059,943.
According to this method, the permeability of methanol can be significantly reduced due to mixing of the inorganic filler and polymer. However, the ion conductivity of the polymer membrane is also considerably lowered.